1. Field of the Invention
The present invention relates to a bar coating method and apparatus and, more particularly, to a bar coating method and apparatus in which a coating liquid having a high viscosity or low wettability is applied.
2. Description of the Related Art
A lithographic printing plate is manufactured as described below. A web of pure aluminum or aluminum alloy is grained, and an anodic oxide film is formed on the surface thereof as necessary, by which a web that serves as a base material is prepared. Further, a photosensitive layer forming liquid or a thermally-sensitive layer forming liquid is applied to the grained surface of the web, and the web is dried to form a photosensitive or thermally-sensitive platemaking surface. In such a manufacturing process, when a coating liquid such as the photosensitive layer forming liquid or the thermally-sensitive layer forming liquid is applied to the web, a bar coater is generally used.
The bar coater is an apparatus in which a bar is brought into contact with a continuously running web, and a coating liquid is supplied to the web feed side through a slit, by which the coating liquid is weighed and applied by the bar. As the type of bar coater, a large number of types have so far been published. Japanese Patent Application Laid-Open No. 2-90972 has disclosed the shape of a weir plate on the coating liquid supply side, and Japanese Patent Application Laid-Open No. 2002-192050 has disclosed a device for preventing an air film accompanying the web surface from being brought to a coating liquid puddle.
In either of the bar coaters, if a coating liquid having a high viscosity is applied to a web running at a high speed, there arises a problem in that a defect such as film breaking occurs on a coating film, and the coating operation is unstable.
Therefore, in Japanese Patent Application Laid-Open No. 2-90972, a bar coating method in which a clearance between the weir plate and the bar is regulated is described as a method for uniformly applying a coating liquid having a medium viscosity to a web running at a speed of about 100 m/min. Also, in Japanese Patent Application Laid-Open No. 2002-192050, a bar coating method in which the weir plate having a certain thickness is provided on the web feed side as a method for applying a coating liquid having a medium viscosity at a far higher speed.